About six years ago I was visiting my sister in South America and she told me about a friend of hers, well really a friend of her husband, who once sat down in the first class section of the plane with a coach seat.
As I boarded the plane back to the states, I thought, why not. If not know then when. So I plopped myself down in the bulk head seat of first class. I was asked to move because someone was assigned there. The stewardess which is now called a flight attendant asked to see my boarding pass. I pretended not to find it and moved to a row further back. A few minutes later I was displaced again, by a PILOT.
I then moved to another row where I sat for the remainder of the flight.
I will never do that again, and not just because I now know that airlines will upgrade loyal members to empty first class seats. Oh well, did it once, but not again.